1. Field of the Invention
The invention relates to a vessel comprising a hull of a substantially closed surface having at deck level a lifting crane, ballast tanks within the hull and a ballast control system for admitting water to the ballast tanks for changing the draft of the vessel.
2. Description of the Related Art
It is known to float offshore constructions, such a support structures for semi-submersible platforms, to their deployment site a relatively shallow draft level, while the semi submersible support structure is towed by one or more tugs. The support structure comprises two parallel, substantially horizontal ballastable buoyancy bodies and vertical support columns resting on the buoyancy bodies. At the deployment site, the support structure is ballasted to a relatively deep draft level, such that a stable configuration with a low centre of gravity is obtained. A superstructure carrying hydrocarbon processing and/or exploration equipment and crew quarters is attached to the columns of the support structure by a crane at a safe distance above water level.
Lifting barges for lifting the superstructures of oil or gas platforms may have a lifting capacity of 1200 tons or over, and need to operate at calm seas. The lifting barges are relatively wide for increased stability. This increased width however makes these barges susceptible to wave moments as the natural frequency of these wide vessels is close the average wave period of about 7-8 s. Hence the known lifting vessels and barges have a limited operating window will be subject to undesired roll motions under all conditions.